Without limiting the scope of the present invention, its background will be described with reference to transmitting downhole data to the surface during a measurement while drilling ("MWD") operation. The principles of the present invention, however, are applicable not only during the drilling process, but throughout the utilization of the fluid or gas extraction well including, but not limited to, logging, testing, completing and producing the well.
In the past, a variety of communication and transmission techniques have been attempted in order to provide real time data from the vicinity of the drill bit to the surface during the drilling operation or during the production process. The utilization of Measurement While Drilling ("MWD") with real time data transmission provides substantial benefits during a (drilling operation that enable increased control of the process. For example, continuous monitoring of downhole conditions allows for a timely response to possible well control problems and improves operational response to problems and potential problems as well as optimization of controllable drilling and production parameters during the drilling and operation phases.
Measurement of parameters such as bit weight, torque, wear and bearing condition on a real time basis provides the means for a more efficient drilling operation. Increased drilling rates, better trip planning, reduced equipment failures, fewer delays for directional surveys, and the elimination of the need to interrupt drilling operations for abnormal pressure detection are achievable using MWD techniques.
At present, there are four categories of telemetry systems have been utilized in attempts to provide real time data from the vicinity of the drill bit to the drilling platform or to the facility controlling the drilling and production operation. These techniques include mud pressure pulses, insulated conductors, acoustics and electromagnetic waves.
In a mud pressure pulse transmission system, resistance of mud flow through a drill string is modulated by means of a valve and control mechanism mounted in a specially adapted drill collar near the bit. Pressure Pulse transmission mechanisms are relatively slow in terms of data transmission of measurements due to pulse spreading, modulation rate limitations, and other disruptive limitations such as the requirement of mud flow. Generally, pressure pulse transmission systems are is normally limited to transmission rates of 1 to 2 bits per second.
Alternatively, insulated conductors, or hard wire connections from the bit to the surface, provide a method for (establishing downhole communications. These systems may be capable of a high data rate and, in addition, provide for the possibility of two way communication. However insulated conductors and hard wired systems require a especially adapted drill pipe and special tool joint connectors which substantially increase the cost of monitoring a drilling or production operation. Furthermore, insulated conductor and hard wired systems are prone to failure as a result of the severe down-hole environmental conditions such as the abrasive conditions of the mud system, extreme temperatures, high pressures and the wear caused by the rotation of the drill string.
Acoustic systems present a third potential means of data transmission. An acoustic signal generated near the bit, or particular location of interest, is transmitted through the drill pipe, mud column or the earth. However, due to downhole space and environmental constraints, the low intensity of the signal which can be generated downhole, along with the acoustic noise generated by the drilling system, makes signal transmission and detection difficult over long distances. In the case where the drill string is utilized as the primary transmission medium, reflective and refractive interferences resulting from changing diameters and the geometry of the connections at the tool and pipe joints, compound signal distortion and detection problems when attempts are made to transmit a signal over long distances.
The fourth technique used to telemeter downhole data to surface detection and recording devices utilizes electromagnetic ("EM") waves. A signal carrying downhole data is input to a toroid or collar positioned adjacent to the drill bit or input directly to the drill string. When a toroid is utilized, a primary winding, carrying the data for transmission, is wrapped around the toroid and a secondary is formed by the drill pipe. A receiver is connected to the ground at the surface where the electromagnetic data is picked up and recorded. However, in deep or noisy well applications, conventional electromagnetic systems are often unable to generate a signal with sufficient intensity and clarity to reach the desired reception location with sufficient strength for accurate reception. Additionally, in certain applications where the wellbore penetrates particular strata, for example, a high salt concentration, transmission of data via EM over any practical distance is difficult or impossible due to ground and electrochemical effects.
Thus, there is a need for a downhole communication and data transmission system that is capable of transmitting data between a surface location and equipment located in the vicinity of the drill bit, or another selected location in the wellbore. A need has also arisen for such a communication system that is capable of operation in a deep or noisy well or in a wellbore penetrating formations that preclude or interfere with the use of known techniques for communication.